This invention relates to a spacer plate for supporting hexagonal fuel element boxes with respect to one another in a sodium-cooled fast breeder nuclear reactor.
Conventionally, several hundred fuel element boxes of hexagonal cross section are clamped together to form a compact bundle in the core of sodium-cooled fast breeder reactors. The clamping is effected generally by means of several clamping rings, for example, of ferritic steel, arranged underneath and above the zone occupied in the boxes by the fuel elements proper. At these two height levels, all the fuel element boxes are in engagement with adjacent ones with the intermediary of spacer plates.
In order to maintain the pulling and pushing forces necessary for replacing the fuel element boxes within permissible limits, a self-welding must not occur at the interfaces of the spacer plates during operation of the reactor. Assuming a radial displacement of one or several fuel elements within the hexagonal box, for example, in the direction of one corner of the fuel element box, due to an outward deformation because of asymmetrical temperature changes, such a displacement causes the box to act as an obtuse wedge. The two fuel element boxes which contact the wedge flanges (that is, which contact those two adjoining sides of a fuel element box which form the wedge), should expediently be capable of executing a reversible yielding displacement. Such a yielding displacement, however, can take place only if the frictional forces at the interfaces of the spacer plates do not exceed a predetermined value. Thus, a self-wedging must not occur in any event. For this purpose, it is known to provide, at different cross-sectional planes of the fuel element bundle, rolling-element bearings (ball bearing, roller bearings or pin bearings) which are arranged between the fuel element boxes for the mutual support thereof. A reduction of friction in this manner, however, involves substantial manufacturing costs. Further, it was found that roller bearings or ball bearings do not function satisfactorily in liquid sodium, since at the contact faces characterizing these bearings excessive pressures are generated and thus a significant wear due to this phenomenon could not be avoided.
Further, spacer plates interposed between fuel element boxes are known whose material has a specific frictional coefficient less than 0.5. These known components are, as planar plates or corner plates made of a homogeneous material, arranged on all six sides of each hexagonal fuel element box and are thus in a mutual contacting relationship. The experimentally determined material for these components has wear and frictional properties which meet the requirements. The cobalt content of this material, however, is so high (65%) that the irradiation of the sodium circuit by the activated wear fragments may pose a real danger. Spacer plate materials which, on the other hand, are free from cobalt or contain only small quantitites thereof, do not have favorable frictional properties if the interfaces are of identical material.